1. Field of the Invention
The invention relates to a high frequency signal detection circuit used in the radio communication device, detecting a very small high frequency signal inputted from an antenna.
2. Description of the Related Art
Japanese patent 2,561,023 discloses a conventional high frequency signal level detection circuit. Under the '023 Japanese patent, the circuit detects a level of a high frequency signal by detecting the signal by a semiconductor diode. Under the circuit, the bias voltage for the use of the detection of the semiconductor diode is changed into three levels periodically. Then, the circuit calculates a static characteristic at that time from the diode currents over two voltages that are the second and the third voltages, which are greater than the suitable electrical potential for the signal detection, and adds the suitable electrical potential on the first voltage and amends an output of the detection by calculating the signal detection efficiency.
As described in the '023 Japanese patent, when the high frequency signal is detected by a semiconductor diode, the signal to be detected is required to be large enough. Thus, a very small high frequency signal received at an antenna should not be applied to a semiconductor diode directly. Accordingly, a high frequency amplifier circuit is generally disposed between the antenna and the detection circuit. However, there is a problem that a high frequency amplifier circuit consumes large power. For this reason, a circuit having a source-follower type amplifier for amplifying an input signal received at an antenna is proposed. However, under the use of the source-follower type amplifier, amplitude of the output becomes smaller than that of the input. Thus, in order to detect a small signal, another amplifier is required to be disposed at the preceding stage or at the subsequent stage.